1. Field of the Invention
The invention relates generally to offshore oil production and, more particularly, to offshore oil storage that can be used for deepwater applications.
2. Background Art
A major factor in determining whether or not to exploit an offshore oil and gas field is the feasibility of handling and transporting the hydrocarbons to market once they are produced. Generally, hydrocarbons produced offshore must be transported to land-based facilities for subsequent processing and distribution. Temporary storage may be provided at the offshore production site for holding limited quantities of hydrocarbons produced and awaiting transport to shore. In some cases, equipment is also provided at the offshore production site for separating and/or treating the produced hydrocarbons prior to storing and transporting them to shore.
In the case of an offshore production facility located relatively close to shore, hydrocarbons produced may be feasibly transported to shore through a pipeline system extending from the offshore site (e.g., offshore platform or subsea wells) to the shore along the ocean floor or seabed. This type of pipeline system is typically preferred, when feasible, because it permits the constant flow of hydrocarbons to shore regardless of the weather or other adverse conditions.
However, in some parts of the world, the use of a seabed pipeline system for transporting hydrocarbons to shore may result in expensive pipeline tariffs.
For offshore facilities located a great distance from shore, construction of a pipeline to shore is typically not practicable. In these cases, floating vessels, known as tankers, are used to transport hydrocarbons to shore. Tankers are specially designed vessels which have liquid hydrocarbon storage (or holding) facilities, typically, in the hull of the vessel. In the case of crude oil production, water, vapor, and other impurities are typically removed from the oil prior to offloading the oil onto tankers for transport. In some cases, tankers include additional equipment for separating and treating crude oil prior to storage and transport.
Because tankers float on the water surface, their operations are largely dependent upon surface conditions, such as wind, wave, and current conditions. Thus, tankers are typically not operated during severe or unfavorable conditions. Additionally, operation of a particular tanker may be interrupted periodically for maintenance and repairs. Due to the large expense associated with maintaining tankers, tankers may also be shared among several offshore sites. As a result, long delay periods may occur between tanker availability for a particular site. Therefore, it is desirable to have storage facilities available at the offshore site to avoid the need to xe2x80x9cshut-inxe2x80x9d (or halt) production due to tanker unavailability. Additionally, offshore storage may be desired to allow for continuous production operations, independent of tanker hook-up and disconnect operations, as discussed below.
Examples of existing offshore production and storage systems used for deepwater applications are illustrated in FIG. 1 and in FIGS. 2A-2D. FIG. 1 shows one example of a production platform 2 used in a deepwater application. This platform 2 includes processing and storage equipment 4 for separating and treating crude oil collected from subsea wells 6 and storing a limited quantity of the processed oil when transport is not available. Because the surface area and weight carrying capacity of the production platform 2 is extremely limited, storage facilities provided on a platform 2 are limited in size and, thus, inadequate for handling large quantities of hydrocarbons which may be produced during periods of shuttle tanker or other hydrocarbon transport unavailability.
FIG. 2A shows a floating production, storage, and offloading (FPSO) system 10 which comprises a tanker 11 specially equipped to function as an offshore production facility. The FPSO tanker 11 is permanently moored at the offshore site and connects to the subsea wells or subsea production gathering system 14 through one or more flowlines 18 connected to the production inlet 16 of the FPSO tanker 11. During production operations, produced hydrocarbons are received, directly or indirectly, from the subsea wells 14. Once on the FPSO tanker 11, hydrocarbons are processed and temporarily stored. Hydrocarbons stored on the FPSO tanker 11 are periodically transferred onto a shuffle tanker 12 temporarily positioned in the vicinity of the FPSO tanker 11 during the transfer. Because FPSO systems 10 comprise surface vessels, they are susceptible to severe weather conditions, during which production must be interrupted and the flowlines 16 leading to the FPSO tanker 11 disconnected. Furthermore, positioning of the shuttle tanker 12 close to the FPSO tanker 11 for hydrocarbon transfer is typically limited to relatively calm weather conditions. As a result, the storage space on the FPSO system 10 may become full and production may have to be halted until a shuttle tanker 12 for offloading is provided.
FIG. 2B shows one example of a floating storage and offloading (FSO) system 20, which is a pure form of ship-based storage without production facilities on board. Using this type of storage system, produced hydrocarbons from a production platform 22 are transferred to an FSO vessel 26 via a flowline (not shown) extending from the production platform 22 to the FSO system 20. Hydrocarbons transferred to the FSO vessel 26 are stored, typically in the hull of the FSO vessel 26. From the FSO vessel 26, produced hydrocarbons are periodically offloaded onto a shuttle tanker 24 for transport to shore. As in the case of the FPSO system 10 discussed above with reference to FIG. 2A, production operations which depend upon an FSO system 20 for storage may be susceptible to production interruptions due to severe weather conditions. Also, during periods when a shuttle tanker 24 is not available for offloading the storage facility on the FSO vessel 26, it may become fall requiring interruption of production until a shuttle tanker 24 is available.
FIG. 2C is an illustration of a direct shuttle loading (DSL) system 30. In a DSL system 30 hydrocarbons produced from subsea wells 33 are collected at an offshore production gathering system, in this case a production platform 32, and directly offloaded onto a shuttle tanker 34, 38 when available, through a flowline 36. For the DSL system shown in FIG. 2C, hydrocarbons are loaded onto one shuttle tanker 34 for transport to shore while another shuttle tanker 38 waits nearby for subsequent offloading after the first tanker 34 is fall and en route to shore. Like other tanker-based storage systems described above, production operations which use DSL systems 30 are susceptible to interruptions in production due to severe weather conditions and periods of shuttle tanker unavailability. Additionally, the use of a DSL system 30 may require operation of a larger shuttle tanker fleet because the presence of at least one shuttle tanker 34, 38 is required at substantially all times in order for production operations to continue. Further, in cases where no temporary storage is provided at the production site, hydrocarbon production will be interrupted every time a shuttle tanker 34, 38 is connected or disconnected for offloading and transport.
Production platforms have also been developed to integrate oil storage into the hull 44 of a platform, such as a SPAR platform 40 as shown in FIG. 2D. However, in cases involving significant production volumes, this storage is not adequate during periods of tanker unavailability. Thus, frequent tanker hook-ups to the platform 40 will still be required. In such cases, even a system comprising a platform 40 with integral storage is still too dependent upon the presence of a shuttle tanker 42.
Other offshore storage systems for deepwater applications may also include smaller thick-walled tanks designed to be sunk to the seabed and internally controlled from the surface. Because the interiors of these tanks are completely isolated from the surrounding seawater environment, these tanks require very thick walls to withstand the hydrostatic pressure difference between the subsea environment and the platform environment. As a result, these systems are expensive and limited in capacity. These systems also require additional equipment such as pumps, controls, and other instrumentation, for monitoring and controlling the internal tank environment and moving fluids in and out of the tanks.
Other offshore storage systems exist for use in shallow water applications; however, for the most part, these systems are not applicable for use in deepwater applications.
In view of the above, a need exists for a cost-effective storage system that can be used for deepwater production operations which provides adequate facilities for storing hydrocarbons and acts as a buffer between tanker loadings. Having such a storage system may avoid the need to halt production until tanker availability and may help to increase the profitability of an offshore production site or to increase the feasibility of developing production sites in remote offshore locations.
The invention relates to a system for storing liquid hydrocarbons, such as oil, in a tank located on a seabed and offloading the stored hydrocarbons from the tank onto transport vessels when they are available for transporting hydrocarbons to shore. Embodiments of the invention may be used in conjunction with an offshore production facility, such as an offshore platform, or a subsea production and processing system. Embodiments of the invention may also, advantageously, provide a more feasible large capacity hydrocarbon storage option, particularly for deepwater hydrocarbon production.
In one embodiment the system includes a storage tank attachable to the seabed and adapted to store hydrocarbons therein. The system also includes at least one fluid channel having a first end positioned inside the tank proximal the bottom of the tank, and a second end in fluid communication with seawater outside of the tank. The system also includes at least one offload line having a first end coupled to and in fluid communication with the tank proximal a top of the tank and a second end adapted to be fluid coupled to a tanker and accessible from a water surface. The system further includes at least one hawser having a first end operatively coupled to the tank and a second end adapted to be accessible from the water surface and attachable to a tanker to anchor the tanker during tanker offtake operations.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.